1. Field of the Invention
There are many nondestructive testing methods, including radiography, dye-check and ultrasonic testing, that are used to examine completed welds.
The present invention relates to the use of acoustic emission to detect flaw formation in welds during welding and cooling phases.
2. Description of the Prior Art
A book entitled, "Research Techniques and Nondestructive Testing" edited by R. S. Sharpe and published in 1970 by Academic Press Inc. (London) Ltd., London, England, includes, as Chapter 1, an article entitled, "Acoustic Emission," that summarizes the state of the art of using acoustic emission to detect and locate flaw growth in pressure containers undergoing hydrostatic testing. It also states that acoustic emission technology has been developed to detect flaw formation in welds during welding and cooling phases and in this regard refers to reports by C. K. Day and W. D. Jolly, that were issued in 1968 as reports BNWL-902 and BNWL-817, respectively. Chapter 1 of the book does not indicate the construction of the system used by Day or Jolly.
Dunegan Research Corporation (now known as Dunegan/Endevco Endevco Corp.) had prior to the present invention, a 3000 series acoustic emission instrumentation system that is constructed to provide for research a number of types of instrument evaluation utilizing acoustic emission. In the instruction manual it states that the totalizer of the system can be used to monitor welds. The system discloses preamplification of signals from a transducer, band pass filtering of the preamplified signals, and post-amplification followed by digital counting after signal conditioning. The digital counting is controlled by a digital reset clock. The digital counting uses a ring-down counting method to measure the energy of acoustic emission pulses. The method of ring-down counting used a non-synchronous time interval during which counting occurs. Because it is possible for an acoustic emission pulse to occur over two counting periods, the count in either period may be indistinguishable from lower energy pulses occurring entirely within the counting period. If the counting period is extended so that the probability of splitting pulses over two periods is reduced, the probability of allowing two pulses within the same period is increased. Optimization is possible, but the probability of counting a pulse correctly is limited to less than 100%. In the case of acoutic weld monitoring this maximum probability is about 80% with standard equipment providing ring-down counting.